The invention relates to a tyre with radial carcass reinforcement anchored within each bead to at least one bead wire, and comprising a crown reinforcement constituted by at least two plies known as working plies, radially superposed and formed of reinforcement elements which are parallel to each other within each ply and crossed from one ply to the next, forming with the circumferential direction of the tyre angles whose value may be of between 10 and 35° in absolute value.
In numerous cases, the Applicant has found that several quality criteria relating to the tyre in question, in particular for use on heavy vehicles, may be improved by the presence in the crown reinforcement, radially between said working crown plies, of an additional ply formed of metallic elements oriented substantially parallel to the circumferential direction.
Mention may be made of application FR 2 744 954 (corresponding to Colom U.S. Pat. No. 6,082,425) which, with a view to reducing the operating temperature of a heavy goods vehicle tyre (tire) by means of an economic and effective solution, recommends using an additional ply of circumferential reinforcement elements having an axial width at least equal to 1.05 times the width of the widest working ply, the reinforcement elements of said additional ply having a larger diameter than the diameter of the elements of the working plies.
In the above context, the additional ply may be formed of continuous steel cables referred to as semi-elastic, that is to say cables exhibiting relative elongation at break of greater than 2%. These cables allow the achievement of a level of rigidity suitable for harmonious distribution of circumferential tension between the working crown plies and the additional ply. Said cables are advantageously described as “bi-modulus”, that is to say exhibiting a curve, representing tensile stress as a function of relative elongation, having gentle slopes for slight elongations and a substantially constant, sharp slope for greater elongations. The very low modulus prior to curing, for elongations of less than 2%, makes it possible to increase the circumferential extent of the additional ply during curing of the tyre.
The additional ply may also be formed of metal cables made of steel oriented circumferentially and cut so as to form sections of a much smaller length than the circumferential length of the ply, the cuts between sections being axially offset relative to one another. Such an embodiment makes it possible to impart simply to the additional ply the rigidity desired, whatever that may be.
The choice of elastic or cut cables for reinforcing the additional ply does not provide said ply with the best resistance to fatigue, respectively as a consequence of a reduction in the breaking load of the elastic cables, or as a consequence of the existence of stress concentrations in the calendering mix of the cut cables.
International application WO 99/24 270 teaches that better resistance to separation between carcass ply and crown reinforcement, and between crown plies, and resistance to fatigue of the circumferential reinforcement elements of the additional ply may be obtained by using as reinforcement elements for the additional ply circumferentially continuous metallic elements undulating in the plane of the ply, the undulations of said elements being parallel to each other, in phase and oriented circumferentially such that the ratio a/λ) of the amplitude a to the wavelength λ decreases axially from the centre to the edges of said ply so as to be minimal at said edges.
From reading the two documents cited above, it is clear that the reinforcement elements of said additional ply are preferably metal cables of steel with a larger diameter than that of the reinforcement elements of the two radially adjacent working plies. Whereas the structure described in the first cited document (use of semi-elastic reinforcement elements or cut elements) impairs the tensile strength and endurance of the additional ply itself, the structure described in the second cited document (undulating reinforcement elements) is difficult to implement and in particular increases manufacturing costs substantially, said additional ply being obtained by helical winding of the reinforcement element around a former until the total width of the ply is obtained.